In A.C. capacitive-type guaging systems for fuel quantity measurement in aircraft, such as that described in U.S. Pat. No. 4,020,691, in which stray capacitive effects from aircraft wiring have been shown to have been eliminated by means of suitable rectification of the A.C. signals and subsequent integration thereof to produce an A.C. signal independent of any stray A.C. pick-up in the circuit path.
There is a need, however, for compensating for manufacturing tolerances in such capacitive-type probes, and while such compensation can be done in stitu, that is, in-aircraft calibration, it still is desirous and of greater importance to have such calibration done prior to installation in the aircraft, that is, to provide a bench calibration method for such capacitive probes. Also, previous known methods for adjusting capacitance of such probes for calibration purposes involved adjusting the dry capacitance prior to installation--by varying a shunt resistance trimmer connected to one of the plate members, for example--would not correct for error in wet capacitance change due to fuel being the dielectric. Similarly, a method of providing a variable "window" in the outer tube or electrode will make a correction in the overall capacitance by changing the profile at one place, thus introducing an error in profile.